Prior patents relating to this invention are U.S. Pat. No. 4,576,050 to Lambert entitled, "Thermal Diffusion Fluid Flow Sensor," and U.S. Pat. No. 4,782,708 to Harrington and Heremans entitled, "Thermocouple Sensors," both of which are incorporated herein by reference.
The Lambert patent is directed to a sensor having a silicon chip covered by a polyimide film which supports a heating resistor for emitting thermal waves and a thermocouple for sensing the thermal waves. When the sensor is immersed in flowing fluid such as air, the thermal waves are propagated from the heating resistor to the thermocouple through the fluid at a rate which is dependent on the flow rate of the fluid. A circuit is provided to supply oscillating current to the heating resistor to emit the thermal waves and to detect the time of arrival of the thermal waves at the thermocouple and determine flow rate on the basis of propagation time. The thermocouple system was proposed to be Pb/PbTe. The Pb/PbTe thermocouple has an output of about 0.5 mv/K but has an unstable resistance.
The Harrington and Heremans patent discloses a thermocouple system comprising BiSb forming junctions with gold, platinum or nickel for use in the Lambert flow sensor. The Pt/BiSb thermocouple has an output of 0.140 mv/K and has a stable resistance, although it is subject to corrosion. With either thermocouple, the signal developed by the thermocouple detector is quite small and requires an expensive high gain, low noise amplifier to couple the detector signal to the timing circuit which determines the propagation time.